Combination water heating and space heating systems, wherein hot water is used for both domestic and space heating purposes, are known in the art. See for example, the combination systems described and shown in U.S. Pat. Nos. 4,371,111; 4,541,410; 4,584,987; 4,641,631; 4,766,883; and 5,544,645. In such combination systems, in response to a demand for heating in an indoor space, heated water is circulated between a hot water storage tank and a hydronic heat exchanger coil located in an air supply duct, to heat air passing over the coil, thereby providing heated air to the space. Heated water in the tank is also used to satisfy a demand for domestic hot water (e.g., showers, laundry, dishwasher, etc.).
If there is a concomitant demand for space heating and for domestic hot water, the hot water supply in the tank may be insufficient to satisfy both demands. Accordingly, priority must be given to either the space heating demand or the domestic hot water demand.
U.S. Pat. No. 4,584,987 teaches a fireplace boiler system for heating water, primarily for space heating use. The hot water is available for domestic use only after a space heating demand is satisfied. Therefore, hot water is not available for domestic use at the same time that there is a demand for space heating.
U.S. Pat. No. 5,544,645 discloses a combination water heating and space heating apparatus in which priority is given to a demand for domestic hot water in the event of a low temperature condition in the hot water storage tank. The control system tries to prevent this problem by anticipating the increased demand for hot water by raising the setpoint of the hot water storage tank in response to a demand for space heating.